Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires gas ion bombardment of a target having a face formed of a material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a heat conducting backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use o an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target race-way.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known dispositions in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
Titanium targets and alloys containing Ti, such as Ti-W alloys are used as cathodic sputter targets to coat a plurality of substrates. Commonly, elemental Ti targets are used to provide contact metals in semiconductors with Ti-W alloys finding wide use as diffusion barriers between platinum silicide contacts and other interconnect metallizations.
Solder attachment of such titanium containing targets to heat conductive backing plate members, such as copper or aluminum backing plates, has proven troublesome due to the poor wettability between the target and tin, indium, and/or lead-based solders.
One approach to the problem has been to apply an anchor or adhesive layer of nickel or nickel aloy on the titanium via plasma spraying processes to provide a wettable surface for the solders. Unfortunately, plasma spraying processes induce a large amount of stress into the titanium, leading to warpage problems. Additionally, the nickel coating readily oxidizes during the heating of the titanium prior to soldering. This resulting oxide is not easily wetted by the solder. Another problem inherent in plasma coatings is unevenness of application, leading to the requirement that the anchor or adhesive layer must be sanded prior to soldering.